As a liquid cooling scheme of cooling electronic units such as servers and supercomputers, immersion liquid cooling is used in which an electronic unit is cooled by being immersed in an insulating coolant stored in a liquid tank.
Immersion liquid cooling is classified into a single liquid phase cooling scheme and a cooling scheme using a gas-liquid phase change.
The single liquid phase cooling scheme is a scheme of cooling an electronic unit with a liquid-phase coolant without vaporization of a coolant.
In this scheme, to avoid the coolant from vaporizing with heat from an electronic component such as a central processing unit (CPU) implemented on an electronic unit, a chemosynthesis oil with a boiling point sufficiently higher than a heat generation temperature of that electronic component is used as a coolant. With the coolant circulated between a heat exchanger and a liquid tank by using a pump, the coolant cooled at the heat exchanger is supplied to the liquid tank, and the electronic unit is cooled in the liquid tank with the coolant.
However, the chemosynthesis coil has high viscosity and thus has low cooling capability. To compensate for the cooling capability, the driving force of the pump has to be increased to speed up circulation of the coolant.
If viscosity is high, it takes time for the coolant to drip down from the electronic unit when the electronic unit is drawn up from the liquid tank for maintenance, posing a problem of poor maintainability.
Meanwhile, in the cooling scheme using a gas-liquid phase change, the coolant is vaporized with heat from the electronic unit and the electronic unit is cooled with heat of vaporization. To allow the coolant to vaporize with heat from the electronic unit, the coolant for use has a low boiling point and tends to evaporate.
The coolant with a low boiling point has lower viscosity compared with the one for use in the single liquid phase cooling scheme. Thus, unlike the above, the driving force of the pump does not have to be increased and, with the coolant dripping down from the electronic unit in a short time, maintainability is favorable.
However, the cooling scheme using a gas-liquid phase change is susceptible to improvement in avoidance of the naturally-evaporated coolant from escaping into the air.
The followings are reference documents.    [Document 1] Japanese Laid-open Patent Publication Nos. 9-162579 and    [Document 2] Japanese Laid-open Patent Publication No 1-264247.